Haskell Esqueleto project subset of columns to list of custom records

Question

In all the examples I have seen the results from esqueleto are projected into a list of tuples or to entities records.

For example:

previousLogItems          


        

Answer 1

What esqueleto is actually doing here is a little complicated. Here is the type for select:

select :: (SqlSelect a r, MonadIO m) => SqlQuery a -> SqlReadT m [r]

This takes an SqlQuery a (a monad wrapping the value you return), and returns an SqlReadT m [r] (a monad wrapping a list of results). When you return your Custom type, the following happens:

1. esqueleto converts your a type to persistent's internal SQL representation
2. persistent executes the query and gets back a results list
3. esqueleto converts the results list from persistent's internal SQL representation to [r]

---

To make this work for custom types, you'll need to instantiate SqlSelect, and define conversion functions to and from the persistent types:

data Custom' = Custom' (Value Text) (Value Int)

data Custom = Custom
  { title :: Text
  , id    :: Int
  }

instance SqlSelect Custom' Custom where
  sqlSelectCols esc (Custom' a b) = (mconcat [ta, ", ", tb], va ++ vb)
    where
      (ta, va) = sqlSelectCols esc a
      (tb, vb) = sqlSelectCols esc b
  sqlSelectColCount _ = 2
  sqlSelectProcessRow [PersistText a, PersistInt64 b] = Right $ Custom a b
  sqlSelectProcessRow _ = Left "Error: Incorrect rows to translate to Custom"

(note that I wasn't actually able to test any of the above, so it might have bugs)

One thing to note is that in the above example, the a and r types are not the same (Custom' vs Custom). This is because inside of a select, all of the values you're working with are of type Value, not their actual types.